British Patent Specification No. 1,088,183 discloses a process for the continuous preparation of iso- or terephthalic acid by the oxidation of m- or p-xylene with a source of molecular oxygen in the presence of a benzoic acid solution of one or more transition metal oxidation catalysts and a source of bromine conducted at a temperature in the range of from 170.degree. C. up to 275.degree. C. and at a gauge pressure in the range of from 21 up to 35 kg/cm.sup.2 with the removal of by-product water as it is formed.
According to U.S. Pat. No. 4,081,464 there were two disadvantages associated with the process of said British Patent and both disadvantages were associated with the assigned operating gauge pressure of from 21 up to 35 kg/cm.sup.2. The first disadvantage, occurring at the start-up of the oxidation, was associated with a sudden somewhat drastic increase in oxidation temperature, a substantially instantaneous increase of about 110.degree. C., which caused over-oxidation of the magnitude of charring of product and solvent as water content of the benzoic acid solvent decreased from 5 weight percent to 0 weight percent as would occur when trying to remove by-product water as rapidly as it was formed. Such drastic temperature increase occurred without an accompanying increase in the pressure in the oxidation zone.
The second disadvantage was associated with continuity of operation after a successful start-up had been accomplished. The manifestation of the second disadvantage was a rather wise cycling of temperature above and below a set selected operating temperature.
Both of said disadvantages were overcome according to U.S. Pat. No. 4,081,464 by conducting the continuous oxidation of m- or p-xylene not in the presence of liquid benzoic acid as the solvent but rather in the presence of a solvent system consisting essentially of from 85 up to 97 weight percent benzoic acid and from 15 down to 3 weight percent water, at a temperature of from 175.degree. C. up to 235.degree. C., at a gauge pressure in the range of from 6 up to 25 kg/cm.sup.2 and by varying the rate of water condensate recycled to the oxidation zone in response to its temperature change from a selected preset temperature so that the rate of water recycled increased or decreased with an increase or decrease from said temperature. By following those operating guides continuous oxidation could be successfully started without an initial drastic temperature increase and smoothly continued with not more than a .+-.5.degree. C. temperature variation from a selected constant operating temperature to consistently achieve the 85 to 95 mole percent yield of iso- or terephthalic acid product of the purity promised by the above British Patent.
However, the iso- or terephthalic acid product recovered from the process of either the above British Patent or U.S. Patent did not have the current exceptionally high purity equivalent to "fiber grade quality" which is indicated by a carboxybenzaldehyde content of not more than 0.015 weight percent (150 ppm). Thus a suitable process was needed for converting to said fiber grade quality the iso- or terephthalic acid products recovered from such processes. The known art pertinent to such purification was surveyed for likely purification techniques to integrate with the oxidation processes of said patent.
There have been proposed many techniques for purifying rather insoluble impure iso- and terephthalic acids. Most all of the proposed techniques start with said acids as solids recovered from an oxidation process. Since about 1959 the proposed purification techniques have started with impure iso- or terephthalic acid product recovered from an oxidation conducted in the presence of acetic acid as reaction solvent. Part of the previously proposed purifications involved esterifying the impure iso- or terephthalic acid with methanol recovering the impure diester product and subjecting the diester to fractional crystallization or distillation. Another portion of the purification techniques involved dissolving the recovered impure iso- or terephthalic acid as a water soluble salt in water, and subjecting the solution to oxidizing and/or reducing agents and then precipitating the free acids by acidifying the solutions so treated.
Still another portion of the pertinent purification techniques involve the catalytic hydrogenation of solutions of the impure iso- or terephthalic acid. Three of such catalytic hydrogenations of most interest were described in U.S. Pat. Nos. 3,546,285 and 3,584,039 and in British Patent Specification No. 1,056,319. The purification techniques of the two U.S. Patents started with solid impure iso- or terephthalic acid whose main impurity was, respectively, 3-carboxybenzaldehyde (3-CBA) or 4-carboxybenzaldehyde (4-CBA) recovered from the catalytic air oxidation of m- or p-xylene in acetic acid reaction medium or solvent or recovered from the nitric acid oxidation of m- or p-xylene. Such recovered impure iso- or terephthalic acid was dissolved either in an aliphatic saturated fatty acid (e.g. acetic acid) containing 0 to 30 percent by volume of water (No. 3,546,285) or in water (No. 3,854,039) and then the solution was subjected to hydrogenation at elevated temperature (generally above 250.degree. C.) under liquid phase conditions and preferably in the presence of supported or unsupported Group VIII noble metal as catalyst. Both references indicate a preference for metallic palladium disposed on the surface of a charcoal as the hydrogenation catalyst. Purified iso- or terephthalic acid product is precipitated by cooling the solution after the solution is separated from catalyst (Pat. No. 3,854,093) or before the solution is separated from the catalyst (Pat. No. 3,546,285). The latter patent also suggests using benzoic acid with 0 to 30 volume percent water in place of the fatty acid with 0 to 30 volume percent water to form the solution for catalytic hydrogenation. Further the same patent demonstrates that cobalt oxidation catalyst retained by the impure phthalic acid has little or no useful hydrogenation catalytic effect with respect to impurity removal.
However, both of the foregoing catalytic hydrogenation purification processes appear to imply that it is desirable to effect some partial purification of iso- or terephthalic acid by its separation from the reaction solvent mother liquor (e.g., acetic acid mother liquor) which causes some of the impurities and most of the oxidation catalyst components to be retained in solution in the mother liquor. But such pre- or partial purification not only adds to the processing time but requires substantial capital expenditure for one or more high pressure and at least one moderate pressure crystallizer, a filter, or centrifuge for accomplishing the solid-liquid separation, drying of recovered reslurrying of the recovered solids, and plant space.
The purification techniques of British Patent Specification No. 1,056,319 is applied directly to the fluid effluent from the oxidation of m- or p-xylene with a source of molecular oxygen containing from 20 percent (air) up to 100 percent (oxygen gas) oxygen in the presence of acetic acid (1 to 10 weight parts acetic acid per weight part of xylene), in the presence of 0.1 to 1.0 weight percent transition metal oxidation catalyst (e.g., Co, Mn or Co and Mn) and 0.1 to 1.0 weight percent bromine based on the xylene at a temperature in the range of from 150.degree. C. up to 230.degree. C. Such fluid effluent, an acetic acid mother liquor, a suspension of iso- or terephthalic acid precipitated during oxidation of the xylene, is heated to a temperature in the range of from 260.degree. C. up to 315.degree. C. under pressure to maintain acetic acid in the liquid phase and in the presence of sufficient acetic acid to dissolve all solids present in said effluent. The weight ratio of acetic acid to xylene of 8 to 10:1.0 used in the oxidation provides said "sufficient acetic acid" but the acetic acid to xylene ratio of 1 to 7:1.0 used in the oxidation requires the addition of acetic acid to the oxidation effluent before heating it to the 260.degree. to 315.degree. C. temperature. Acetic acid mother liquor or its acetic anhydride fortified product having about 5 to 10 weight percent water content is disclosed as preferred for dissolving the solids at the 260.degree. to 315.degree. C. temperature.
The resulting 260.degree. to 315.degree. C. temperature solution is then with or without prior treatment with hydrogen at 15 to 200 pounds per square inch hydrogen partial pressure cooled to a temperature in the range of from 150.degree. C. to 176.degree. C. to precipitate crystalline iso- or terephthalic acid for recovery. The recovered crystalline product is washed with acetic acid and dried.
Such direct redissolving and recrystallization is demonstrated to cause an 85% decrease in 4-CBA in the absence of hydrogen treatment and a 91% decrease in 4-CBA with hydrogen treatment of the solution prior to recrystallization.
The concept of the process of British Pat. No. 1,056,319 might be integratable with the oxidation processes of the foregoing British Pat. No. 1,088,183 or U.S. Pat. No. 4,081,464 and eliminate the extra capital costs and process time noted before providing the recrystallization or hydrogenation and recrystallization from the liquid benzoic acid solvent of the British Patent or the benzoic acid-water solvent of the U.S. Patent could be perceived as providing a product of purity equivalent to fiber grade quality. To perceive success for such direct purification routes would further require knowledge of the relative solvent properties, of liquid benzoic acid or the benzoic acid-water solvent system vis-a-vis the solvent properties of acetic acid containing 5 to 10 weight percent water for the catalyst components and impurities produced during the oxidation and their reduction products.
It was determined in our laboratories that the solvent properties of liquid benzoic acid and the 85-97% benzoic acid--15 to 3% water solvent system for dissolving iso- or terephthalic acid to be too limited at commercially feasible processing temperatures and pressures. It was also determined that a solvent system containing from 25 up to 75 weight percent water and 75 to 25 weight percent benzoic acid could dissolve suitable, from process economic and design basis, amounts of iso- and terephthalic acids at commercially feasible temperatures. The solvent system containing 25 to 75% benzoic acid and the balance water would, at said feasible temperature, require to maintain liquid phase conditions operating pressures slightly below the pressure required when water was the only solvent component because the mole fraction of benzoic acid is quite small (less than 0.01) in the mixture of water and benzoic acid vapors in equilibrium above such liquid solvent.
The saturation temperatures for 10, 20, 30, 40 and 50 grams of terephthalic acid per 100 grams of 25% water-75% benzoic acid, 50% water-50% benzoic acid and 75% water-25% benzoic acid are shown in TABLE I wherein benzoic acid is shown as "BA."
TABLE I ______________________________________ SATURATION TEMPERATURES, .degree.C. Terephthalic Water-Benzoic Acid Solvent Systems Acid, grams/100 25% H.sub.2 O-- 50% H.sub.2 O-- 75% H.sub.2 O-- grams solvent 75% BA 50% BA 25% BA ______________________________________ 10 246 231 232 20 274 257 255 30 293 273 268 40 303 282 278 50 313 292 285 ______________________________________
The saturation temperatures for 10 to 50 grams of isophthalic acid per 100 grams of the water-benzoic acid solvent systems consisting of 25, 50 and 75 weight percent water will be found to be about 100.degree. C. lower than the saturation temperatures for the same concentrations of terephthalic acid in the same solvent systems.